The present invention generally relates to magnetic reproducing apparatuses having a capstan servo circuit, and more particularly to a magnetic reproducing apparatus having a capstan servo circuit which controls the rotation of a capstan so that noise bar in a reproduced picture appears at a constant position on the reproduced picture and will not move during a high-speed reproducing mode in which a video signal is reproduced from a magnetic tape which travels at a tape speed which is higher than the tape speed upon recording.
In a magnetic recording and reproducing apparatus which records and reproduces a video signal onto and from tracks which are formed obliquely on a magnetic tape with respect to the longitudinal direction of the magnetic tape, recording positions of a vertical synchronizing signal of the video signal must be aligned at end parts of the oblique and parallel recording tracks upon recording. On the other hand, upon reproduction, rotary heads must accurately scan over the above recording tracks. Hence, in this type of a so-called helical scan type magnetic recording and reproducing apparatus (hereinafter simply referred to as a magnetic recording and reproducing apparatus), a head servo circuit and a capstan servo circuit are generally provided. The head servo circuit comprises a speed control loop for controlling the rotational speed of the rotary heads, and a phase control loop for controlling the rotational phase of the rotary heads. The capstan servo circuit controls the rotational speed and the rotational phase of a capstan which drives the tape to travel.
During a normal reproducing mode of the magnetic recording and reproducing apparatus of the above type, the rotational phase of the rotary heads is synchronized to a reference signal by a phase error signal. This phase error signal is obtained by comparing the phase of a detection signal which is detected in accordance with the rotation of the rotary heads by the phase control loop of the head servo circuit, with the phase of the reference signal. Further, the rotational phase of the capstan is synchronized to the above reference signal by another phase error signal. This other phase error signal is obtained by comparing the phase of a control pulse which is reproduced by a phase control loop of the capstan servo circuit, with the phase of the reference signal. Accordingly, the phase of the reproduced control pulse, that is, the rotational phase of a capstan motor, is in synchronism with the rotational phase of the rotary heads.
On the other hand, a high-speed search reproduction can be performed in the above magnetic recording and reproducing apparatus, to search for a predetermined recording position on the magnetic tape. During the high-speed search reproducing mode, the magnetic tape travels in the same direction as upon recording (forward direction), or in a reverse direction, at a high speed which is in the order of seven times the tape speed during the normal reproducing mode, for example. Because the magnetic tape travels at a high speed in the forward or reverse direction during the high-speed search reproducing mode as described above, the relative scanning linear speed between the rotary heads and the magnetic tape during the high-speed search reproducing mode is different from the relative scanning linear speed during the normal reproducing mode. Accordingly, the horizontal scanning frequency of the reproduced video signal during the high-speed search reproducing mode, is different from the regular horizontal scanning frequency of the reproduced video signal during the normal reproducing mode. Thus, if the reproduced signal obtained during the high-speed search reproducing mode is supplied to a television receiver, the reproduced picture will be out of horizontal synchronism, and a regular reproduced picture cannot be obtained.
Therefore, in order to obtain a reproduced picture which is in horizontal synchronism even during the high-speed search reproducing mode, it becomes necessary to carry out reproduction so that the horizontal scanning frequency of the reproduced video signal becomes the same as the horizontal scanning frequency during the normal reproducing mode (15.734 kHz when the video signal is an NTSC system color video signal, for example). In order to achieve this, the rotational speed of the rotary heads must be increased or decreased during the high-speed search reproducing mode, as compared to the rotational speed of the rotary heads during the normal reproducing mode, according to the direction of the tape travel. That is, if the tape traveling direction during the high-speed search reproducing mode is the same as the tape traveling direction (forward direction) during the normal reproducing mode, the rotary heads must be rotated at a rotational speed which is higher than the rotational speed of the rotary heads during the normal reproducing mode. On the other hand, if the tape traveling direction is in the reverse direction during the high-speed search reproducing mode, the rotary heads must be rotated at a rotational speed which is lower than the rotational speed of the rotary heads during the normal reproducing mode. Hence, in the head servo circuit described before, the phase control loop is cut off during the high-speed search reproducing mode, and moreover, a voltage obtained by subjecting the horizontal scanning frequency of the horizontal synchronizing signal to a frequency-to-voltage conversion, is applied to a motor (hereinafter referred to as a drum motor) for rotating the rotary heads. Hence, the rotational speed of the rotary heads is controlled so that the relative scanning linear speed between the rotary heads and the magnetic tape during the high-speed search reproducing mode is the same as the relative scanning linear speed during the normal reproducing mode.
However, because the phase control loop of the head servo circuit is cut off during the high-speed search reproducing mode, the rotation of a drum motor, that is, the rotation of the rotary heads, is out of phase with the reference signal which is obtained from a reference oscillator during this mode. On the other hand, the capstan motor is rotated at a high speed, in phase with the reference signal, during the high-speed search reproducing mode. Hence, the control pulse reproduced from the magnetic tape which is driven to travel by the capstan, and the rotation of the rotary heads, become out of phase during the high-speed search reproducing mode.
On the other hand, during the high-speed search reproducing mode, scanning loci of the rotary heads on the magnetic tape deviate from the recorded tracks, because the magnetic tape travels at a high speed during this mode. Thus, the rotary heads successively scan obliquely across the adjacent tracks. For this reason, if a guard band is formed between the adjacent tracks, noise is generated as the rotary head scans across this guard band. If the adjacent tracks are formed contiguously by two rotary heads which have gaps of mutually different azimuth angles, noise is generated as the rotary head scans over the track which was recorded by a head having a gap of a different azimuth angle. The generated noise appears as noise bar on the reproduced picture.
However, as described before, the rotation of the capstan motor and the rotation of the drum motor are out of phase during the high-speed search reproducing mode. Thus, the position where the rotary head scans across the guard band, and the position where the rotary head scans across the track which was recorded by the head having the gap of the different azimuth angle, respectively are not constant. Therefore, the position on the reproduced picture where the noise bar appears, is not constant and moves upward or downward. This upwardly or downwardly moving noise bar on the reproduced picture is irritating to the eyes of the viewer, and there was a problem in that the reproduced picture obtained during the high-speed search reproducing mode was unpleasant to watch.
Accordingly, in order to eliminate the above problem, a system may be devised for making the rotation of the rotary heads and the rotation of the capstan motor in phase with each other, by directly comparing the phase of the detection signal which is detected in accordance with the rotation of the rotary heads in the head servo circuit, with the phase of the control pulse which is reproduced in the capstan servo circuit. However, in this devisable system, the circuit construction becomes different for the normal reproducing mode and for the high-speed reproducing mode. Thus, this devisable system is disadvantageous in that a large number of switching circuits are required, and the circuit construction becomes complex. On the other hand, another system may be devised for making the rotation of the rotary heads and the rotation of the capstan motor in phase with each other, by directly comparing the phase of the detection signal which is detected in accordance with the rotation of the rotary heads in the head servo circuit, with the phase of the control pulse which is reproduced in the capstan servo circuit, even during the normal reproducing mode so as to simplify the circuit construction. However, according to this latter devisable system, the unstable rotation of the rotary heads during the normal reproducing mode will give undesirable effects on the capstan servo circuit, and there is a disadvantage in that the rotation of the capstan motor will become unstable.